Pixel response‐based EPID dosimetry for patient specific QA

Abstract Increasing use of high dose rate, flattening filter free (FFF), and/or small‐sized field beams presents a significant challenge to the medical physics community. In this work, we develop a strategy of using a high spatial resolution and high frame rate amorphous silicon flat panel electronic portal imaging device (EPID) for dosimetric measurements of these challenging cases, as well as for conventional external beam therapy. To convert a series of raw EPID‐measured radiation field images into water‐based dose distribution, a pixel‐to‐pixel dose–response function of the EPID specific to the linac is essential. The response function was obtained by using a Monte Carlo simulation of the photon transport in the EPID with a comprehensive calibration. After the raw image was converted into the primary incident photon fluence, the fluence was further convolved into a water‐based dose distribution of the dynamic field by using a pregenerated pencil‐beam kernel. The EPID‐based dosimetric measurement technique was validated using beams with and without flattening filter of all energies available in Varian TrueBeam STx™. Both regularly and irregularly shaped fields measured using a PTW 729 ion chamber array in plastic water phantom. The technique was also applied to measure the distribution for a total of 23 treatment plans of different energies to evaluate the accuracy of the proposed approach. The EPID measurements of square fields of 4 × 4 cm2 to 20 × 20 cm2, circular fields of 2–15 cm diameters, rectangular fields of various sizes, and irregular MLC fields were in accordance with measurements using a Farmer chamber and/or ion chamber array. The 2D absolute dose maps generated from EPID raw images agreed with ion chamber measurements to within 1.5% for all fields. For the 23 patient cases examined in this work, the average γ‐index passing rate were found to be 99.2 ± 0.6%, 97.4 ± 2.4%, and 72.6 ± 8.4%, respectively, for criterions of 3 mm/3%, 2 mm/2%, and 1 mm/1%. The high spatial resolution and high frame rate EPID provides an accurate and efficient dosimetric tool for QA of modern radiation therapy. Accurate absolute 2D dose maps can be generated from the system for an independent dosimetric verification of treatment delivery.


| INTRODUCTION
The use of amorphous silicon (aSi) flat panel electronic portal imaging device (EPID) for online and offline dosimetric verification has been sought after over the years by several research groups and industrial companies. [1][2][3][4][5][6] For example, the Portal Dosimetry TM from Varian Medical Systems (Palo Alto, CA, USA) has been available for pretreatment QA. 1 In this product, beams are directly applied to the portal imager and time-integrated imaging data are acquired. By comparing the measurement with the calculation using the photon fluence from the treatment plan, a QA decision is made based on a series of criteria, such as the percentage difference, distance to agreement (DTA), and  7 and Liu et al. 8 used the approach for pretreatment verification QA of VMAT. Asuni et al. 9 and Lee et al. 10 used EPID images to reconstruct in vivo 3D dose for Stereotactic Body Radiation Therapy (SBRT) QA. Recently, Nelms et al. 11 and Bailey et al. 12 investigated the use of EPIDose TM (Sun Nuclear Corporation, Melbourne FL, USA) for pretreatment QA. The EPIDose converts an EPID image to dose in water by convolving with an experimentally determined kernel to account for the difference in dose-deposition kernels of the EPID and water. Because the detailed EPID response was not studied, for each MLC-segmented field, an output correction factor must be calculated from MLC plan data and applied to the measurement, which may be a significant source of inaccuracy. Greer et al. 13 developed an EPID-based dose prediction model by incorporating MLC leaf effects for IMRT applications. The EPID dose kernel was calculated using an experimental method and is only specific to the Pinnacle treatment planning system. Warkentin et al. 14 improved the approach with a convolutionbased calibration procedure, in which the physics response of the EPID was deduced from the combination of a Monte Carlo-simulated dose deposition kernel in the EPID phosphor, and an empirically derived kernel describing optical photon spreading. Nicolini et al. 15 had recently demonstrate the feasibility of using EPID dosimetry for flattening filter free (FFF) photon beams by means of the GLAaS methodology to validate it for pretreatment quality assurance of volumetric modulated arc therapy (VMAT), but EPID calibration data were obtained against ion chamber measurements. While all these studies indicated that the EPID is useful as a dosimetric tool, to the best of our knowledge, a complete and accurate method to convert MV photon beams physics response of the EPID to a water-equivalent dose distribution has yet to be obtained with consideration of the generation and transport of the optical photons in the scintillators. Furthermore, there are little investigation adapting EPID for dosimetry of a high dose rate and small field radiation therapy (RT). This work is thus devoted to develop a strategy of using a high spatial-resolution and high frame rate a-Si EPID for dosimetric verifications of various modalities of modern RT, including small FFF fields with high dose rate. In the next section, we introduce the setup of experimental data acquisition and the calibration of the system. The methods to deconvolve the primary fluence and waterbased dose are presented in Sec. 2B-C. Validation and application issues related to the implementation of the proposed method are discussed in Sec. 2D-E. We conclude in Sec. 4 with highlights of the study and future perspectives of EPID-based dosimetric verification. tem. In a stationary setting (Fig. 1a), the EPID was placed on the treatment couch, useful for QA measurement of fixed gantry deliveries such as IMRT or any other type of dynamic treatment with the gantry angles of the contributing beams reset to 0°. In the latter case (Fig. 1b), a customized holder was used to mount the EPID on the linac head. The system is capable of measuring the dose at each gantry angle for a rotational arc delivery such as VMAT. For both settings, the EPID imager is placed beneath a 2-cm thick Plas-ticWater â (Computerized Imaging Reference System Inc, Norfolk, VA, USA) build-up phantom for photon measurement. A source to detector distance (SDD) of 100 cm was used for both stationary and rotational settings.

2.A | Overall system setup and data acquisition
Before image acquisition, a dark field (DF) image and a flood field (FF) image were acquired for offset and gain corrections. The offset correction took into account the dark current of each pixel and acquired with photon beam off. In order to create the offset correction image, an averaged image (EPID DF ) of 300 frames of DF images had to be acquired and EPID DF would be subtracted from the incoming pixel data during acquisition time. To homogenize differences in pixel sensitivities, an FF gain correction was carried out at all available photon energies of 6 MV, 10 MV, 15 MV, and 6 MV FFF, 10 MV FFF beams by irradiating the EPID with the incident photon beam fully covering the entire detector-sensitive field (20 9 20 cm 2 ). To create the FF image, an averaged image (EPID FF ) of 300 frames of offset-corrected images has to be acquired. Each EPIDmeasured raw image is corrected by using the following equation The standard flood-field correction method has the effect of removing some beam profiles from the EPID images, such as "horns" induced by the flattening filter. A beam profile correction matrix was generated by using the field measurement data from water scan measurement data with open beam. Delivery with different total monitor unit and different dose rate were also tested in a previous study. 16 The results exhibited good MU linearity and the dose rate dependency was found to be less than 1%.

2.B | Conversion of the EPID raw images to incident photon fluence
To determine the incident photon beam fluence, it was necessary to simulate and calibrate the EPID device to establish a relationship between EPID pixel values and radiation dose. Detailed structure (a) (b) The (a) on-couch stationary and (b) on-head rotational settings for the EPID system.
The dose-glare kernel K dp (x, y) of all available WFF (a) and FFF (b) photon energies for deconvolution of EPID-measured raw images into incident primary photon fluence. With the detail EPID modeled using GATE, a deconvolving kernel K de (x, y) was generated. The incident photon fluence Ψ p (x, y) on EPID can thereafter be reconstructed from the corrected EPID raw image and the K de (x, y) using the flowing equation 2.C | Conversion of the reconstructed incident fluence to water-based relative dose distribution In practice, a water-based dose distribution is measured using different detectors such as an ion chamber, diode, or film with plastic water phantoms for routine dosimetry measurements and patient-specific for all available photon energies were simulated in the MCNPX and specific number of source photons was selected to ensure an acceptable level of statistical uncertainty (< 1% at 3 cm off pencil beam, < 3% at 10 cm off axis for each simulation). The incident photon fluence map Ψ p (x, y) reconstructed from EPID raw measurement was then convolved with the K pb (x, y) to generate a two-dimensional (2D) relative dose distribution in water at different d max depths using D w ðx, yÞ ¼ W p ðx, yÞ K pb ðx, yÞ The photon beams of the TrueBeam STx linac used in this study were calibrated to deliver 1 cGy/MU at the depth of dose maximum (d max ) under reference setup condition of a 10 9 10 cm 2 reference field with a nominal SSD of 100 cm. To determine the absolute doses, 100 MU (100 cGy) was delivered to the EPID with the same reference setup for cross calibrations. The EPID-measured image data was recorded as D EPID and an absolute calibration factor F ABS for each energy was then calculated by calculating the ratio of 100 cGy and These calculated F ABS were used to convert the EPID-reconstructed relative dose into a water-based absolute 2D dose.
All simulations in this study were run on a Linux server computer with 64 cores AMD Opteron central processing units (CPUs) and 128GB random-access memory (RAM). A typical run generally took 2-4 h to yield statistically acceptable results without any effort on acceleration.   show the change in K dp (x, y) as a function of distance from the central axis for WFF and FFF beams, respectively.

2.D | System validation via standard fields
The MCNPX simulated pencil-beam dose kernels K pb (x, y) converted EPID images to 2D dose distribution in water as described in Sec 2C and the results are shown in (Fig. 3) for all available energies.

3.B | System validation via standard fields
In Fig. 4, EPID-measured output factors of different field sizes are shown along with that obtained using Farmer chamber. Overall, the output factors of square fields obtained using the two approaches agreed within 0.85%. The average discrepancy was found to be 0.02% AE 0.46% (mean AE standard deviation), 0.24% AE 0.53%, 0.10% AE 0.40%, À0.16% AE 0.56%, and 0.25% AE 0.59% for 6, 10,

| CONCLUSION S
We have developed an EPID-based dosimetric system based on the use of a Monte Carlo-generated pixel response of the system. The EPID-measured absolute dose distribution and output factors for standard square fields ranging from 4 9 4 to 15 9 15 cm 2 were found to agree well with ion chamber data. The off-axis measurement of the EPID was also found to be consistent with PTW729 and water scan data. For the clinical cases with various field sizes, the agreement between EPID-and PTW729-measured values were found to be better than 2.1%. The success of EPID-based system was also supported by the c index analysis. The proposed EPID dosimetric system addresses an important unmet clinical need for an efficient and reliable dose measurement and verification in modern RT.